The light weight and strength of magnesium and magnesium alloys makes products fashioned therefrom highly desirable for use in manufacturing critical components to be used, for example, for aircraft, for terrestrial vehicles or for electronic devices. But the most significant disadvantage of magnesium and magnesium alloys that they easily corrode. The exposure of such metallic materials' surfaces to a chemically hazardous environment causes that their surfaces corrode rather quickly and strongly. Corrosion is both unesthetic and reduces strength.
There are many methods known for improving the corrosion resistance of a workpiece of magnesium and magnesium alloy by modifying the surface of the workpiece. It is generally accepted that the best corrosion resistance for magnesium and magnesium alloy surfaces is achieved by anodizing. In the anodizing process, a metallic workpiece is used as an anode or with an alternating current as an anode and as a cathode alternating according to the frequency of the alternating current of an electrical circuit, the circuit including an electrolyte bath in which the workpiece is at least partially immersed. Depending on the properties of the current, the bath temperature and the composition of the solution of the electrolyte bath, the surface of the workpieces may be modified in various ways. The metallic workpiece (substrate, article) may be a coil, a sheet, a wire, a workpiece made from a coil respectively from a sheet or a more or less massive part with a simple or complex shape.
Various solutions and additives are found for example in; U.S. Pat. No. 5,792,335 discloses ammonia and phosphate containing electrolyte solutions with an optional content of ammonium salt and peroxide; U.S. Pat. No. 6,280,598 teaches electrolyte solutions that may contain different amines or ammonia and phosphate or fluoride and subsequently a sealing agent may also be applied; WO 03/002773 describes electrolyte solutions containing phosphate, hydroxylamine and alkali metal hydroxide. The anodizing methods disclosed in these publications allow a layer comprising magnesium hydroxide and magnesium phosphate. These anodizing processes offer high corrosion resistance.
Although anodizing is effective in increasing the corrosion resistance, the hardness and the scratch resistance of the surfaces are often insufficient especially for anodizing coatings generated on the surface of magnesium rich material, primarily because a high concentration of magnesium hydroxide in the generated anodizing coatings. In conventional anodizing processes even on the surfaces of materials rich in aluminum, beryllium, iron or titanium, the generated anodizing coatings are typically rich in at least one hydroxide and therefore not as hard as expected. On the other hand, the processes of anodizing based on acidic electrolyte solutions do not offer a sufficiently high corrosion resistance.
One of the ways to solve this problem is to apply a coating rich in ceramic oxides especially by micro-arc electrolytic oxidation process.
The investigation of micro-arc electrolytic oxidation for light metals has continued for more than fifty years. The micro-arc oxidation method has several names: Micro-arc oxidation, micro-plasmic oxidation, plasma-liquid coating, etc. Methods and compositions to apply a ceramic oxide coating by anodizing on aluminum have been disclosed in several publications: SU 1200591 teaches to build an oxide coating with high hardness and wear resistance in alkaline solutions of potassium hydroxide, “liquid glass” (=water glass) and sodium aluminate. An alternating current with a frequency of about 50 Hz and with a current density in the range from 0.5 to 24 A/dm2 (current density of the cathodic phase) and in the range from 0.6 to 25 A/dm2 (current density of the anodic phase) is supplied to the metallic material. DE 42 09 733 teaches an anode-cathode oxidation in an alkali metal silicate or in an alkali metal aluminate electrolyte solution. Pulses with a frequency in the range from 10 to 150 Hz are used. The method offers solid oxide coatings with a thickness in the range from 50 to 250 microns and requires a very high energy consumption and a complex equipment. U.S. Pat. No. 5,616,229 discloses a method of obtaining a ceramic oxide coating on aluminum. The method uses again potassium hydroxide and silicate in the electrolyte solution.
A general drawback of alkali metal hydroxide and silicate containing electrolyte solutions is the low stability of the said electrolyte solutions. By applying the typical electricity for such a process, the electrolyte solution changes within a short time—especially after the use from about 30 to about 90 A·h/L to a kind of gel because of the high polymerization of the solution and should therefore be completely replaced.
U.S. Pat. No. 4,659,440 teaches a method of coating aluminum articles in electrolyte solutions comprising an alkali metal silicate, a peroxide, an organic acid and a fluoride. A vanadium compound may also be included for decorative purposes. U.S. Pat. No. 5,275,713 discloses a method of coating aluminum surfaces with an electrolyte solution containing alkali metal silicate, an organic acid, potassium hydroxide, a peroxide, a fluoride and molybdenum oxide. The voltage is first raised to 240 to 260 V and then increase the voltage to a range from 380 to 420 V. U.S. Pat. No. 5,385,662 teaches a method of producing oxide ceramic layers on barrier layer-forming metals which include aluminum or magnesium rich metallic surfaces. The electrolyte solutions contain ions of phosphate, borate and fluoride.
A main drawback of the described electrolyte solutions described in these publications is the content of hazardous components like fluorides and heavy metals.
RU 2070622 and U.S. Pat. No. 6,365,028 disclose methods for producing ceramic oxide coatings on aluminum in electrolyte solutions comprising an alkali metal hydroxide, an alkali metal silicate and an alkali metal pyrophosphate. An alternating current with a frequency in the range from 50 to 60 Hz is supplied to the metal. The addition of pyrophosphate ions to the classic combination of alkali metal hydroxide and silicate improves the stability of the electrolyte solution. In order to accelerate the oxide layer formation, the inventor used peroxide additives in the second patent publication mentioned here. A drawback of the disclosed method is the high content of the alkali metal hydroxide that is undesirable for magnesium rich surfaces because of high contents of magnesium hydroxide in the generated coatings.
A high content of an alkali metal hydroxide in the electrolyte solution accelerates the formation of magnesium hydroxide and magnesium oxide on the metallic surfaces and assists in producing coatings with a low hardness and with a low stability against acids. Additionally, a significant content of at least one metal hydroxide seems to reduce the stability of the silicate containing electrolyte solutions severely. U.S. Pat. No. 4,978,432 teaches to produce protective coatings that are resistant to corrosion and wear on magnesium and magnesium alloys. The electrolyte solutions comprise ions of borate or sulfonate, phosphate and fluoride or chloride. The obtained coatings include magnesium phosphate and magnesium fluoride and optionally magnesium aluminate that offer good corrosion and wear resistance. However, the electrolyte solutions are not sufficiently environmentally friendly.
A method that is similar to the proposed invention is disclosed in SU 1713990. It teaches a method of micro-arc anodizing for metals in alkaline electrolyte solutions. The anodizing is performed by an asymmetric AC current so that the hardness is increased because of a good sintering. The current density is decreased by steps in the range from 20 to 60%. The disclosed compositions which include sodium hexametaphosphate (Na6P6O18) do not include a second phosphorus containing compound and no addition of any alkali metal hydroxide. A main drawback of the disclosed method is the complex electrical control and the low rate of the coating formation. The method has not been adapted and not optimized for magnesium rich surfaces.
WO 03/002773 discloses a method of anodizing magnesium surfaces in alkaline phosphate solutions. The method allows to build quickly anodizing layers that contain a magnesium phosphate. The generated layers offer excellent corrosion resistance and good adhesion. The coating method was approved for application in aircraft industries. However, the coatings have a low hardness because of a high content of magnesium oxide and magnesium hydroxide.
It would be highly advantageous to have a method for treating the surfaces of anodizable metallic materials and especially magnesium or magnesium alloy surfaces so as to generate coatings of a high hardness and of a high corrosion resistance. Further on, it is preferable that such a treatment is environment friendly and does not include a considerable content of fluorides, heavy metals and other hazardous components. It would be favorable if this process would be not too complex and not too expensive.